Stamping is a form of metalworking that uses a trim punch and a die trim section mounted to a stamping press to perforate holes or shapes in a part, or blanks out a part using trim punches. The material is placed over a die trim opening and the trim punch is rapidly pushed through the material along a punch stroke having a predetermined length defined by the stamping press and into the die trim profile opening. The stamping press is conventional apart from the die trim section disclosed herein and so the stamping press will not be described in any detail. A slug is stamped from the material. When stamping materials with a trim punch and a die, loose slugs can be tilted or pulled up and into the working area of a stamping die by the extraction stroke of the trim punch. This can cause the in-feeding material to jam, which can cause damage to the parts being made. Additionally, a slug that is pulled into the working area of the stamping die can damage or break the die tooling, which is expensive to repair or replace.
To prevent slugs from being pulled up by the retracting trim punch, it is known to form a recess or groove in the sidewall of the die trim opening. It is also known to modify the cutting edge of a die trim opening by machining grooves across a top surface of the die trim insert or by creating irregularities on the cutting edge of the die trim opening.
Kramski U.S. Pat. No. 4,543,865 discloses forming a uniform longitudinal groove extending parallel with the trim punch stroke into the sidewall of the die trim opening. The length of the groove from the top of the die trim opening is equal to the depth of insertion of the trim punch into the die trim opening. The slug will initially be formed with a small projection or nose caused by a shearing action of the slug material in the groove area. As the trim punch reaches maximum insertion into the die trim opening, the slug is pushed beyond the longitudinal extent of the groove. This causes the trim punch to shear off the projection as the projection re-engages the die trim opening sidewall beneath the groove. The shear creates resistance between the slug and the smooth wall surface of the die trim opening sidewall that resists upward movement of the slug at the start of the trim punch extraction stroke.
The groove found in the Kramski die trim sidewalls have disadvantages.
In Kramski's groove design it is critical that the trim punch stroke extend precisely to the bottom of the groove. This critical relationship between the trim punch stroke depth and the groove depth must constantly and precisely be maintained during routine resharpening of the trim punch and the die trim section.
If the trim punch stroke were too shallow, the projection will not engage the die trim opening sidewall beneath the groove. There would be no shearing action created between the slug and the smooth wall surface to resist upward movement of the slug at the start of the trim punch extraction stroke.
A trim punch stroke that is too deep, on the other hand, would cause the trim punch to push the projection beyond the bottom of the groove. Friction on the slug that would resist upward movement of the slug is decreased because of slug material erosion by the sidewall. The projection will be torn off or sheared off of the slug by the trim punch as the slug engages the sidewall below the groove. Small fragments of the torn-off projection will accumulate in and pack into the groove, causing erosion of the projection material before the projection material engages the opening sidewall. The fragments can be drawn upwards by the retracting trim punch to accumulate in the working area of the die, causing damage to the die tooling or the stamped parts.
In addition, Roberts U.S. Pat. No. 6,397,715 states that Kramski's groove design is expensive and difficult to machine.
Roberts discloses forming multiple grooves or irregularities around the circumference of the cutting edge of the sidewall of the die trim opening. The multiple grooves form multiple non-uniform burrs on the slug. The burrs are formed by slug material tearing away from the remainder of the base material in the vicinity of the irregularities, and are not formed by the normal shearing action between the trim punch and the die trim section. The vertical depth of the irregularities into the die trim opening is less than the depth of insertion of the trim punch. At the bottom of the trim punch stroke the burrs lodge against the smooth wall surface of the die trim opening sidewall. Friction force generated between the burrs and the smooth wall surface resists upward movement of the slug at the start of the trim punch extraction stroke.
The grooves found in Roberts' sidewalls also have disadvantages.
Because a burr is created by the slug material stretching, thinning, and then tearing, a burr deforms relatively easily. The retention force generated by the burr and the smooth wall surface is therefore relatively low.
Furthermore, burrs in any stamping operation are undesirable. Burrs create dirt (small fragments of material that have broken off the base material or slug) in the working area of the die. These dirt fragments can be drawn upwards with the trim punch into the working area of the die. Roberts creates additional burrs on the stamping base material that are carried through the working area of the die. Broken off burrs can damage the die tooling or the stamped parts.
Pushing the burrs created by Roberts' grooves deeper along the smooth surface of the die trim opening sidewall causes slug material erosion that reduces the retention force between the slug and the die trim opening sidewall. This increases the likelihood the slug will be unable to resist upward movement at the start of the trim punch extraction stroke.
Roberts' grooves and irregularities also increase the costs to maintain the die tooling. The grooves and irregularities must be re-machined into the top surface of the die trim section after each and every resharpening of the trim section.
It would be desirable to have a slug retention groove that extends longitudinally parallel with the trim punch stroke that does not have to precisely match the trim punch stroke depth and can generate more friction force between the slug and the die trim insert wall to resists upward movement of the slug at the start of the trim punch extraction stroke. The slug retention groove should not create undesirable burrs or dirt, and should not increase the cost to maintain sharp die trim sections.